Expression of the ribosomal proteins S14, S16, L13a and L30 is regulated by cytokinin and abscisic acid: Implication of the involvement of phytohormones in translational processes

By employing the differential display method we have isolated a cDNA clone for rps14 from lupine, which encodes the ribosomal protein (r-protein) S14. The polypeptide consists of 150 amino acid residues with a molecular mass of 18.8 kDa [Bot. Acta 111 (1998) 287]. Northern analysis revealed that the mRNA level of rps14 responds rapidly, i.e. within 5 h, to phytohormone treatments: the level is stimulated by benzylaminopurine (BA), a synthetic analog of natural cytokinin, and repressed by abscisic acid (ABA). A comparable regulation was observed for the homologous gene in Arabidopsis. In contrast, hormone effects on the S14 protein level could only be detected after 48 h. Western analyses discovered two fractions of the S14 protein in lupine cotyledons: a BA-independent membrane-associated pool and a BA-stimulated soluble pool. S14 is predominantly found in young leaves and hypocotyls. Comparable hormone regulation was found for the mRNA of the ribosomal proteins S16 of the small (40S) subunit as well as for L13a and L30 of the large (60S) subunit of the cytosolic ribosomes. The phytohormone effects on the expression of r-proteins will be discussed in the context of their possible functions in protein synthesis.